This invention relates to a method and apparatus for draining waste matter such as sludge, e.g. from communal wastewater treatment plants, by which method and device the sludge is compressed between two endless movable perforated bands, such as braided wire webs, partially bearing against each other in providing so high as degree of dryness of the sludge, that burning of the sludge will be possible.
Draining of sludge from communal wastewater treatment plants mainly takes place by aid of three different types of apparatus; namely centrifuges, band presses or filter presses. Since sludge normally contains large quantities of water, one has to drain it at the place of collection to avoid otherwise very expensive transportation costs. It is only the filter presses of the above mentioned apparatus which provide an acceptable dryness.
Said filter presses have however a drawback, that they are complicated as to their construction, which also means that they are not safe to operate and difficult and expensive to maintain. Thus the personal contribution in using filter presses is not of an acceptable order of greatness. To that must be added, that working near to such apparatus is not absolutely safe owing to the risk of infection.
Centrifuges are precision apparatus, which rotate with a very high angular speed. They are also very sensitive in that the load is kept on a constant level. In practice, this is not the case since sludge varies in mixture and in type over the course of time. This makes it difficult to control the centrifuge in obtaining an optimum result, and satisfactory results are not normally accomplishable in practice. To that must be added the problems resulting from sensitivity to external interferences and influences as well as maintenance requirements. The dryness levels attained in reality also lie on an unacceptable level of 15-20%.
Band presses of different types are usually relatively simple machines suitable to use in wastewater treatment plants. Therefore they have rapidly grown in acceptance and are now used in a widespread manner. Since compression normally takes place in nips formed between two rotating cylinders, high compression pressures necessarily have to be used since the compression times are relatively short or the number of compression nips are too great. If the compression pressure is increased over a certain value, the flocculations and great parts of the sludge pass out with the water through the wire. To that must be added the whole remoistening problem, in that an increase of the number of compression nips above a certain optimum number does not cause any additional improvement in dryness. The designers' experiments to surmount these problems have resulted in a variety of more or less complicated apparatus. Existing installations of apparatus in this field therefore have become more and more complicated and thus less adapted for its field of applications without obtaining improvements in dryness worth mentioning. The dryness is still in the range between 15-20%, depending on the type of sludge being processed.